Sending Email from Another Address in Outlook
Action
To send an email as another account from your own account, follow the steps below.
In order to send email from an address other than your own, you will first need to have "Send As" permissions for that address.
Instructions
Begin a new email in Outlook.
Click From, and then choose Other E-mail Address...:
In the window that pops up, select From... to bring up the Global Address List.
Type in part of the address you wish to send from and click Go to search the Global Address List. Select the sending address you are looking for and click OK.
This will fill that address into the previous window. Click OK to accept.
You will now see the address you chose in the From field. Continue creating your email and send it as normal when ready.